Mating in Drosophila melanogaster leads to many changes in female behavior and physiology, which are induced by male accessory gland proteins (ACP) transferred to females during copulation (Chapman et al., 1995; Soller et al., 1997; Wolfner, 2002; Kubli, 2003). Mated females show reduced sexual receptivity and a substantial increase in egg production. Mating also shortens the lifespan of female flies; this effect is often ascribed to increased reproductive effort, which may carry nutritional and metabolic costs (Reznick, 1985; Salmon et al., 2001; Kirkwood, 2005). A recent report demonstrated that mating substantially increases food consumption in D. melanogaster females (Carvalho et al., 2006), suggesting that reduced lifespan of mated females may not be a consequence of nutrient shortage as previously postulated. To obtain further insights into the physiological basis of differences in longevity between virgin and mated females, we tested their resistance to starvation and to oxidative stress.
In the first experiment, we compared the survival time of Canton-S virgin and mated females exposed to starvation at different ages. At every age tested, mated females were significantly more resistant to starvation than their virgin counterparts (Fig. 1). In virgin females, starvation resistance showed a general declining tendency as females aged. In mated females, starvation resistance increased with age reaching a maximum on Day 15 and then declined in older females. Thus, starvation resistance is a dynamic trait that depends on both mating status and the age of females. Increased starvation resistance in 15-day-old mated females was observed also in other fly strains. Mated w1118 females survived under starvation 74% longer than virgins (n = 114 and 116 for mated and virgins, respectively, P < 0.0001). Mated Oregon-R females displayed the same trend (not shown), suggesting that increased survival under starvation in mated and reproducing females may be a common trait in D. melanogaster.
The increase in starvation resistance in mated females prompted us to verify that our Canton-S flies show expected trends in fecundity and longevity. We compared these traits in mated and virgin females reared in identical experimental conditions as those tested for starvation resistance. Three cohorts of virgin and mated females had their fecundity recorded for 15 days. Virgin females (n = 140) produced on average 52 ± 6 eggs while mated females (n = 128) produced 141 ± 5 eggs per female over this period; this difference was highly significant (P = 0.0012). Longevity assessment of females used in this experiment showed that the lifespan of virgins was extended by 31%. Mated females had a significantly shorter average lifespan of 45.6 ± 2 days than virgin females (59.7 ± 1.2 days). Thus, mated females used in our study showed an expected decrease in longevity and increase in fecundity; yet, their starvation resistance was significantly higher than in virgins. These data fail to support a previously suggested positive correlation between starvation resistance and longevity, and a negative correlation between starvation resistance and fecundity in D. melanogaster (Service et al., 1985; Chippindale et al., 1993; Simmons & Bradley, 1997).
The ability of mated females to survive longer under starvation may be related to a number of factors, such as lower energy expenditure or higher energy reserves. To probe the physiological bases of increased starvation resistance, we first tested whether mated flies become less active than virgins when starved. Recordings of locomotor activity of mated and virgin females as described by Lee & Park (2004) suggested that both were highly active from the onset of starvation until their death (data not shown). Next, we compared food intake between virgin and mated females using diet containing FDA blue dye no. 1 (McCormick, Hunt Valley, MD, USA), allowing the volume of consumed diet to be estimated (Edgecomb et al., 1994). Mated females had a significantly higher concentration of blue dye in their gut homogenates than did virgin females (data not shown). These data are in agreement with a recent report demonstrating increased food consumption in mated D. melanogaster females (Carvalho et al., 2006).
In the next experiment, we asked whether mated females have higher energy reserves, as this could provide a basis for increased starvation resistance (Djawdan et al., 1998). We compared nutrient levels in groups of females collected at 0, 36 and 60 h after the onset of starvation. Triacylglycerols (TAG), glycogen and proteins were separated and measured as described in Van Handel (1965) and Zhou et al. (2004). Before starvation, TAG content in mated females was nearly twice as high as in virgins (Table 1). During food deprivation, TAG levels declined in both mated and virgin females; however, mated females had retained substantial TAG levels after 60 h of starvation in agreement with their longer survival (Fig. 2a). In contrast to fat levels, prestarvation levels of glycogen were not significantly different between mated and virgin females and declined in both by 36 h of starvation (Table 1). Total prestarvation protein content was not significantly different between mated and virgin females (Table 1). Thus, increased starvation resistance in mated females appears to depend on selective accumulation of lipids, which occurred despite a threefold increase in egg production.
|Nutrient||Before starvation||Starved 36 h||Starved 60 h|
|Triacylglycerols||55.2 ± 1||108.4 ± 10*||21.2 ± 0.7||69.8 ± 4.6*||11.1 ± 2||45.2 ± 6*|
|Proteins||199.0 ± 31||178.6 ± 34||202.9 ± 24||165.0 ± 30||223.4 ± 34||176.2 ± 39|
|Glycogen||18.2 ± 1||19.0 ± 2||2.1 ± 0.3||1.3 ± 0.6||0.6 ± 0.5||0.4 ± 0.2|
In parallel to starvation resistance, we tested resistance to oxidative stress in mated and virgin 15-day-old Drosophila females with two experiments. In the first experiment, flies were placed in Petri dishes supplied with 100 µL of 30% hydrogen peroxide (H2O2) on filter paper. Average survival was significantly lower for mated females compared to virgins (Fig. 2b). In the second experiment, flies were placed in vials containing 5% sucrose on filter paper and kept in an air-tight chamber under 100% oxygen with a flow rate of 1 L per min. Average survival under hyperoxia was again significantly lower for mated females compared to virgins (Fig. 2c). Taken together, our experiments demonstrate that the response to two different stressors, starvation and oxidation, changes in opposing directions after mating. These results are surprising as resistance to multiple stressors was shown to change in parallel in flies selected for greater stress resistance or in longevity mutants (Lin et al., 1998; Harshman et al., 1999; Wang et al., 2004). However, most previous experiments were performed on males, and none compared the responses of virgin and mated females to both types of stress in one study.
Our data provide interesting insights into the poorly understood longevity differences between mated and virgin Drosophila females. Virgin females, which have a longer lifespan, appear to be on self-imposed caloric restriction; they are lean and have higher resistance to oxidative stress. Both limited intake of nutrients and high resistance to oxidative stress have been linked to increased longevity in Drosophila and other model organisms (Partridge & Gems, 2002; Sohal, 2002; Lim et al., 2006). Higher fat levels in mated females may increase their susceptibility to oxidative stress; links between lipid oxidative damage and accelerated aging have been demonstrated in Drosophila (Zheng et al., 2005). Interestingly, the increase in body fat during reproduction has been documented in vertebrate females, including mammals (Naismith et al., 1982; Gosman et al., 2006). Our data showing elevated fat reserves in reproducing flies suggest that this phenomenon may be a conserved evolutionary trait. The switch in metabolic pathways that leads to fat accumulation likely evolved to ensure reproductive success. However, this switch carries a cost of decreased resistance to oxidative stress, which is likely to underlie the shortened lifespan of mated females.